Actually what looks to be happening is that /etc/default/apache-htcacheclean is being included/sourced after DAEMON_ARGS has already been set. The init script sets some HTCACHECLEAN_* variables, then DAEMON_ARGS, and then eventually /etc/default/apache-htcacheclean is sourced. However, DAEMON_ARGS is never reset using the newly sourced ENV vars so they're never used (see bash -x debug output below).

As a workaround, instead of changing init script, you can also add DAEMON_ARGS to bottom of your /etc/default/apache-htcacheclean. Here's my DAEMON_ARGS, which I took directly from the init script: